For example, in an image forming apparatus, such as an electrophotographic copying machine, a printer, and a facsimile machine, an image on an original document is converted into an electric image signal in accordance with a density of image information, based on which an electrostatic latent image is formed on a photosensitive body using a laser beam or the like. Then, the electrostatic latent image is developed into a developer image and transferred onto a sheet, after which the developer image is fused with heat generated by a heater of a thermal fuser to be fixed thereon. The heater (hereinafter, referred to as fusing heater) of the thermal fuser is provided as a load. A heater lamp, such as a halogen lamp, is used as the fusing heater, and a heating resistor or the like is used as a heat source. The fusing heater is provided inside a pair of fusing rollers for nipping and transporting a sheet on which the developer image is to be fused. More specifically, one or more than one fusing heater each having a power ranging from some hundreds Watts to approximately two thousands Watts is provided inside either or both of the pair of fusing rollers. In case of a high-speed image forming apparatus, a fusing heater having a larger capacity is used. Further, the pair of fusing rollers are kept at a predetermined temperature by controlling a power supplied to the fusing heater using a fusing heater's ON/OFF signal which is generated based on a detection result of a temperature sensor provided in such a manner to touch the surface of the pair of fusing rollers.
When the image forming apparatus has a large load to which a power is supplied under control, a large rush current passes through the load at the moment the power supply begins. In the following, how the power source voltage drops as a large current passes through the load will be explained using a halogen heater of the fuser as an example and with reference to FIG. 15. As indicated by a curve (a), when a heater signal comes ON state, a power is supplied to the halogen heater from a commercial power source. Since a resistance value of the halogen heater varies with its own temperature, if a current has not been supplied to the same, the halogen heater has quite a small resistance value, generally 1/10 of a resistance value when heated. When a power is supplied to the halogen heater having a small resistance value, a rush current I.sub.1 flows into the halogen heater as soon as the power is supplied as indicted by a curve (c). The halogen heater is heated as the current flows in and a temperature of the same rises and so does the resistance value. As the resistance value rises, the current flowing into the halogen heater reduces and eventually converges to a normal current I.sub.0, and the halogen heater resumes to a normal state. A ratio of the rush current I.sub.1 to the normal current I.sub.0, I.sub.1 /I.sub.0, ranges from several to ten times. In case of the drawing, since the fusing heater is controlled to start to light on substantially at a zero crossing point of a power source voltage waveform, the rush current can be suppressed to a relatively small value.
As indicated by a curve (b) in the drawing, the rush current flowing into the halogen heater in the above manner causes a voltage drop .DELTA.V.sub.1 around an outlet of the commercial power source that supplies a power to the image forming apparatus or in the other internal lines because of its own impedance. The curve (b) in the drawing represents an envelop of a wave height value of the voltage waveform when the voltage drops. After the current passing through the halogen heater has converged to the normal current, the voltage drop also converges into a small value .DELTA.V.sub.2. When the power supply to the halogen heater is cut, the voltage recovers an original voltage level V.sub.0.
Particularly, since the above rush current causes a significant voltage drop instantaneously, peripheral equipments or lighting equipments may be affected adversely. For example, when a voltage supplied to the lighting equipment drops, there occurs a phenomenon referred to as a flicker, in which illuminance is lowered instantaneously. Recently, to suppress this phenomenon (hereinafter, referred to simply as flicker(s)), devices that consume a large power with respect to the power source are regulated by the flicker test. The flicker test checks whether a voltage in the power source side is dropped below a predetermined level by the load provided in the apparatus. In case of the image forming apparatus, the flicker test is carried out in two modes using their respective regulation limits: a copy mode (the flicker test in this mode is referred to as short flicker) and a standby mode (the flicker test in this mode is referred to as long flicker).
To suppress the flickers, Japanese Laid-open Patent Application No. 242644/1994 (Tokukaihei No. 6-242644) discloses a method referred to as a softstart using a switching technique, in which a conduction angle at which a current passes through the load is increased step by step. When a power is supplied to the load like the above-mentioned halogen heater with the softstart, a current waveform is distorted and noises occur in a wide frequency bandwidth. This causes a malfunction of peripheral electronic equipment or imposes adverse effect on the same. To eliminate the above problem, the apparatuses are also regulated by another type of test referred to as the harmonics test which checks whether harmonic components (actually 2nd through 40th harmonics are checked, and hereinafter collectively referred to as harmonic noises) of the current waveform are within the regulation limit. The safety regulation requires the image forming apparatus to pass the harmonic test, in other words, to maintain the harmonic noises within the regulation limit in a normal copy mode.
Various countermeasures are proposed to clear these regulations. For example, aforementioned Japanese Laid-open Patent Application No. 242644/1994 (Tokukaihei No. 6-242644) also discloses a technique (generally known as the softstart) for suppressing the occurrence of the rush current by increasing the conduction angle step by step with a softstart circuit employing a bidirectional thyristor (also known as TRIAC). When this technique is adopted, the harmonic noises are produced in such a large quantity that an expensive noise filter must be provided to the power source line, thereby increasing the cost undesirably.
The technique disclosed in Japanese Laid-open Patent Application No. 242644/1994 (Tokukaihei No. 6-242644) relates to the method for reducing flickers generated while an entire system is operating by giving a time difference to each of a plurality of heaters to start the conductance at different times separately, and further by passing a current with the softstart. However, in the softstart, the harmonics are also generated by increasing the switching conduction angle to each heater lamp per half cycle from the conduction on the first half cycle in the conventional manner. Since this type of control method controls the flickers and harmonic noises of each heater lamp by the conventional switching, an expensive noise filter can not be omitted to reduce the harmonics.
As has been explained, the flickers are created by a rush current occurring at the start of devices (motors, lamps, etc.,) that need an important appeal current during a short time. When a solution adopted to reduce the rush current by instantaneously switching (phase control) the voltage on the device is implemented, a regularly increasing voltage is applied to the device, which reduces both the rush current and flickers. However, this solution generates the harmonic noises. The longer the switching, the lower the flickers and the higher the harmonic noise level.